G Suite is a renaming by Google of what used to be called "Google Apps". https://en.wikipedia.org/wiki/G_Suite
G Suite competes against Microsoft Office 365, but there are big differences between these services.

Personally, I would recommend directly purchasing G Suites from Google unless a certified and trained reseller is assisting you with professional business services associated with the account. Does "gsuites.org" have administrative control over your account, or do you? Did they claim to be part of the Google G Suites reseller program?

My gut feeling from what you've said suggests that they may just be reselling accounts on a domain that they own, rather than being a full G Suite reseller. I'm not completely certain, but I don't believe the "This account is managed by..." appears if you've got your own G Suite account, even if it's being provided by a reseller.

I'm also a bit concerned that they don't seem to appear in Google's Cloud Partner directory, although they might be listed under another name (I searched for GoEz, Zambroni, and The Six Group and nothing came up). However, none of the partners listed in Miami are at the same address either.

I also find it weird that they want an eBay username on their signup form.

One way to double-check this is to try to login to admin.google.com using your account. While a G Suite reseller will often have access to your admin console as well, you should have primary access, and in fact depending on the reseller relationship, you may even be able to disable their access. See Google Reseller access to Resold domains. If you can't login here at all, then you're basically just an end user on their account.

Personally, I'd say that unless they can provide confirmation that they're authorized by Google to be doing whatever it is that they're doing (which in most cases, would require them to be an official Google Cloud Partner), I'd steer well clear. For the buck a month in price difference, you're far better off to just sign up for G Suite directly, as Bill suggests, especially since if they're just reselling mailboxes from their own account, they're in violation of the G Suite Terms of Service, which means that you're taking the risk that Google could shut them down at any point.

It does look fishy when I'm not allowed access to admin.google.com as you suggested. It says 'admin.google.com is for G Suite accounts only. Regular Gmail accounts cannot be used to sign in to admin.google.com'

In this case I believe they have access to every mail I receive and every photo I store with this account. I decided to never use it because it is too risky.

Yup, as I suspected it sounds like they've got their own G Suite account (which anybody can set up directly with Google), and they're just selling sub-accounts off of it. This is definitely against the G Suite Terms of Service, so even aside form the fact that they do technically have access to everything in your account, there's also a high risk of Google shutting them down.

As Bill points out, you can go and get your own G Suite account from Google for $5-$10/month per user (depending on the service level you want), which would be the exact same thing these guys are offering, except that you'd be the one managing it, and it would be legit.

My two cents--don't bother with any but well-known and rated services that you can research on places like here. To me, at least, saving even $50 a year is not worth it when one lost email might mean a missed job opportunity, or missing a credit card warning, etc. Email is too important.